In recent years, liquid crystal displays (LCDs) have come into widespread use as display devices for various types of imaging applications and for products such as personal computers and television sets. There is also the expectation that liquid crystal displays will find further use in stereoscopic 3D imaging as that technology comes into wider use.
However, because of the poor response characteristics of the liquid crystal itself, the LCDs have the potential problem of poor response time. In a typical display device such as used in a television or video imaging application the display is refreshed at a frame rate of 60 frames per second or every 16.7 milliseconds. Higher frame rates of 120 frames per second and 240 frames per second corresponding to 8.3 ms and 4.16 ms respectively are also becoming more common. The purpose of the higher frame rates is to reduce the effects of motion blurring when rapidly changing scenes are presented to the viewer. However, the higher frame rates can only be effective if the liquid crystal can be made to respond in correspondingly shorter times.
The term “optical response time” as used in the industry refers to the time needed for the luminance on the screen of a liquid crystal display (LCD) to rise from 10% of luminance to 90% of luminance. This term also can be used to describe luminance decay from 90% luminance to a 10% luminance. The decay time is typically different than the rise time. The luminance percentages are calculated by first measuring the total difference between the final value and the starting value of the luminance.
Some solutions to these poor response time problems with LCDs are disclosed, in for example, U.S. Pat. No. 6,778,160 B2.
A shorter response time for moving pictures provides a clearer contour or, in other words, less blurred edges. The blur has an additional independent cause and that is the holding time of a picture in a frozen state during the frame time Tf. This patent application focuses on the response time but has to take into account the implications of the holding time.
A further advantage of a short optical response is the full luminance being displayed longer during the frame time yielding a brighter picture or a lower and hence more power-saving backlight.
The long enough presentation of the full luminance during the frame time Tf is the harder to realize the shorter the frame time which is needed to reduce blur. This is depicted in FIG. 1. Therefore shorter frame times also necessitate shorter response times.
The decay of the luminance was for a long time given by the relatively long relaxation time of the LC-material in the area of 25 ms. It could be shortened to around 1 ms by the introduction of an additional electric field [1]. This fact is already included in the decay in FIG. 1 together with the short rise time. The consequence of short rise times and short decay times is that the luminance at the end of the frame time can become zero without sacrificing a long time of full luminance. The zero luminance at the end of the frame eliminates the need for a very costly frame memory for the initial condition of the next frame.
Therefore, the invention as described in this patent is intended to reduce the optical response time so as to reduce blur, enhance luminance, and reduce the power dissipation of the backlight while eliminating the need for a costly frame memory.
Specifically, the desired target for a shortened response time is approximately 0.2. to 0.4 milliseconds. This is calculated for TV systems with a frame rate of 240 Hz. This frame rate is needed to improve image quality for fast moving images and also to be able to present 3D images. The frame time for 240 Hz is 4.16 milliseconds. If it is desired to achieve full black to white luminance in approximately 20% of this time then the response of the LC cell should be no more than 0.8 milliseconds. However, for smaller luminance changes the driving voltage will be less and the response time will have to be correspondingly shorter with 0.2 to 0.4 milliseconds as a desirable target value.
Prior art methods have tried to accomplish this improved response time by a number of different techniques. One is to align the LC molecules with a slight pre-tilt of around seven degrees. This avoids having zero initial torque on the molecules when an electric field is applied. However, due to manufacturing variations this technique is not fully successful in achieving response times below about 30 milliseconds.
Another method involves fringe field switching and requires a structure where there is an edge to the electrode in each pixel. This can result in limitations to the optimal pixel layout and from achieving maximum luminance. This method, however, has been shown to shorten response times to below approximately 15 milliseconds.
A third known method is to apply an offset voltage of 1 volt to 2.5 volts to the black level. This voltage is kept below the threshold where gray shades would begin to appear. However, this method does not further reduce the response time for modulation between gray levels.
A fourth known method is to reduce the cell gap spacing from the conventional range of approximately 3.5 to 5 microns to the range of 2 microns. However, the disadvantage of this technique is that there is a reduction in manufacturing yield and therefore increased cost because of the tighter tolerances for cell spacing.
A further known technique is to try to boost the voltage within the addressing time according to FIG. 4. However, there is typically not enough addressing time available for television frame rate displays. In a typical TV display with 1080 rows presented with a 60 Hz frame rate, the addressing time per row is only 15.43 microseconds, which is too short to be effective.
Applying a combination of these methods can result in shortened response times in the approximate range of 5 milliseconds. This is still insufficient compared to the desired response time of approximately 0.2 to 0.4 milliseconds so as to achieve the correct luminance response between gray levels in sequential frames.
To overcome these limitations, a special signal processing circuit is described herein that is included within each pixel and applies a modified voltage beyond the addressing time but within each frame time.